Abra, particularly early in game if you're trying to catch one or defeat it for a lot of experience, because you either don't have a sleep/paralysis attack or it outspeeds you and runs away.

Golbat, because there will be times when you just want to get through a cave and avoid all wild battles, and this particular Pokemon won't let you escape (Seems to be a secret natural power of all wild Golbat, strangely).

Basculin, because wherever I surfed, I would encounter nothing but Basculin in BW; so many that I quickly grew sick and tired of them. Even Tentacool and Wingull weren't ever this bad! Hopefully they aren't such a nuisance in BW2.